Sailing vessels require a number of blocks for raising, lowering and adjusting sails. A typical block includes a head, a shackle adapted to secure the head to the boat mast or deck, an axle, a structural frame coupling the head to the axle and a sheave journaled around the axle carrying a line connected to the sail. The turning sheave is typically protected by side cheeks which partially enclose the sheave.
Strength, weight and frictional resistance are the primary design factors for the principal blocks in competition sailing vessels. The objective is to have a block with maximum strength, minimum weight and minimal frictional resistance when loaded.
Because of the marine environment, corrosion factors impose very significant limits affecting design of such blocks. Specifically, corrosion determines of the types materials that can be used alone or in combination with others for constructing marine blocks. Corrosion is the result of electrochemical reactions in sea water environments and to a lesser degree in fresh water environments. Corrosion affects both performance and capacity of a block. For example, the effect of corrosion on the bearing surfaces between the sheave and the axle can significantly increase frictional resistance of the sheave rotating around the axle. Galvanic or "two metal" corrosion is even more catastrophic causing the bearing surfaces to unexpectedly seize, and structural components to suddenly fail upon repeated stress loads (corrosion fatigue).
Frequently, materials which by themselves or in combination should theoretically be very resistant to corrosion, corrode rapidly in sea water environments because of trace elements and/or contaminants within the material. For example, Aluminum alloys because of an oxide (Al.sub.2 O.sub.3) film are considered relatively corrosion resistant and are frequently used in marine environments. Yet Aluminum alloys typically contain trace amounts of copper which, in a sea water give rise to numerous electrochemical corrosion reactions. Also, Aluminum is relatively high (electropositive) in the Activity Series (electromotive series) and will dissolve into and displace most other structural metals from electrolyte solutions. Hence in a sea water environment where abrasion removes the protective AL.sub.2 O.sub.3 film coating, the aluminum alloys readily react electrochemically in a classic galvanic fashion.
For strength, typically the shackles, heads, axles and structural frames of marine blocks are formed from high strength structural materials such as steel capable of withstanding tensile loading. Sheaves are composed of light weight materials capable of withstanding compressive loading e.g. aluminum alloys. The side cheeks not being subjected any loads have not been composed of a structural material.
The bearing surfaces between the axles and frames or the axle and sheave typically determine the maximum functional load bearing capacity of a block. For example, a block that can structurally support substantial load ( e.g. above 15,000 lbs.) is useless if the sheave does not rotate when it is loaded. Further, friction between the bearing surfaces of the sheave and axle conventionally increases in direct proportion to the load supported. While, classically, lubricants are used to diminish the frictional resistance between bearing surfaces, not many lubricants are effective or last very long in the harsh environment of the sea. Further, the hardened steels from which bearings and bearing surfaces are typically made with the capacity to carry extremely high static and dynamic loads (in excess of 15,000 lbs) without significant deformation corrode rapidly in sea water.
Considering the forgoing parameters, in 1981 David E. Schramm, one the applicants here, designed a series of marine blocks for sailing vessels which included cast stainless steel heads and stainless steel axles, aluminum sheaves and cheeks, and a pair of exterior titanium/titanium alloy straps connecting between the head and the axle to provide the structural frame.
Mr. Schramm utilized a filament wound epoxy glass bushing with a Teflon liner of the type manufactured by GARLOCK BEARING, INC. of Throfare, N.J. to provide the radial bearing surfaces between the a rotating sheave and stationary axle. This particular type of bushing was chosen because it is essentially inert in the sea water environment and because Teflon inherently becomes a better lubricant on smooth steel surfaces diminishing both static and rolling friction as load increases.
The exterior straps composed of titanium or one of it alloys were chosen because of their very high strength, low weight and resistance to corrosion particularly in sea water. (Titanium ranks 35th among the elements in terms of content in sea water with 1.1 kg. of titanium per cubic kilometer.)
One of the primary problems discovered with the series of marine blocks so designed and developed by David E. Schramm related to galvanic corrosion at the aluminum/steel interfaces, a factor which required all aluminum/steel contact to be eliminated.
Another significant problem discovered related to the rigidity of the frame structure in which the turning sheave was anchored. In particular, lateral support to mechanically prevent the sheave from wobbling as it rotated could not be effectively provided. Such wobble resulted from lateral static loading across the radial bearing and resulting deformation of that bearing under extreme load conditions.
The series of marine blocks so designed and developed by David E. Schramm were identified as the 900 Series Low Profile Blocks, the 1000 Series Blocks, the 2000 Series Blocks and the 2000-6 Series Blocks and since 1981 have been manufactured for and sold by MARINER COMPANY located in Southern, Calif.